docs: add missing content-type header for image load

Without a content-type header, Go uses a max post size
of 10 megabytes, which makes loading images fail
if they're larger than 10 megabytes

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/api/docker_remote_api_v1.18.md

@@ -1843,6 +1843,7 @@ See the [image tarball format](#image-tarball-format) for more details.
 **Example request**
 
     POST /images/load
+    Content-Type: application/x-tar
 
     Tarball in body
 

docs/reference/api/docker_remote_api_v1.19.md

@@ -1924,6 +1924,7 @@ See the [image tarball format](#image-tarball-format) for more details.
 **Example request**
 
     POST /images/load
+    Content-Type: application/x-tar
 
     Tarball in body
 

docs/reference/api/docker_remote_api_v1.20.md

@@ -2068,6 +2068,7 @@ See the [image tarball format](#image-tarball-format) for more details.
 **Example request**
 
     POST /images/load
+    Content-Type: application/x-tar
 
     Tarball in body
 

docs/reference/api/docker_remote_api_v1.21.md

@@ -2219,6 +2219,7 @@ See the [image tarball format](#image-tarball-format) for more details.
 **Example request**
 
     POST /images/load
+    Content-Type: application/x-tar
 
     Tarball in body
 

docs/reference/api/docker_remote_api_v1.22.md

@@ -2608,6 +2608,7 @@ See the [image tarball format](#image-tarball-format) for more details.
 **Example request**
 
     POST /images/load
+    Content-Type: application/x-tar
 
     Tarball in body
 

docs/reference/api/docker_remote_api_v1.23.md

@@ -2653,6 +2653,7 @@ See the [image tarball format](#image-tarball-format) for more details.
 **Example request**
 
     POST /images/load
+    Content-Type: application/x-tar
 
     Tarball in body
 

docs/reference/api/docker_remote_api_v1.24.md

@@ -2666,6 +2666,7 @@ See the [image tarball format](#image-tarball-format) for more details.
 **Example request**
 
     POST /images/load
+    Content-Type: application/x-tar
 
     Tarball in body
 

docs/reference/api/docker_remote_api_v1.25.md

@@ -2672,6 +2672,7 @@ See the [image tarball format](#image-tarball-format) for more details.
 **Example request**
 
     POST /images/load
+    Content-Type: application/x-tar
 
     Tarball in body