Change the plural race name for merfolk to Merfolk

See issue #2940.

[ci skip]

(cherry-picked from commit 296894c515)

changelog.md

@@ -46,6 +46,8 @@
    * Added team color to a few background units missing in Aethermaw.
  ### Packaging
    * OpenMP support has been removed. It is no longer an optional build-time dependency.
+ ### Units
+   * Changed the plural name for the merfolk race from Mermen to Merfolk (issue #2940).
  ### User Interface
    * Swapped the position and formatting of game names and titles in the MP lobby.
    * Made Faction Select button's purpose more clear in MP Staging.

data/core/units.cfg

@@ -253,7 +253,7 @@ The Dunefolk's inquisitive and explorative nature does not preclude military str
         id=merman
         male_name= _ "race^Merman"
         female_name= _ "race^Mermaid"
-        plural_name= _ "race^Mermen"
+        plural_name= _ "race^Merfolk"
         # wmllint: local spelling piscine dextrous
         description= _ "Something like a fusion between humans and fish, the merfolk are an enigmatic race with both piscine and humanoid attributes. They have strong tails that lend themselves to quick movement in any watery environment while their dextrous hands and intelligent minds allow fine craftsmanship and toolmaking. Semi-aquatic by nature, merfolk can breathe both water and air without difficulty. Despite being able to survive on land, they are much quicker and more agile in the water and will rarely be found far from the ocean. They are typically wary of dry land, as they are awkward and clumsy there and they struggle greatly to move over rough or forested terrain."
         num_traits=2