Fix missing apostrophes in docs

Author: slix

docs/api.md

@@ -184,7 +184,7 @@ information on how to create port bindings and volume mappings.
 
 The `mem_limit` variable accepts float values (which represent the memory limit
 of the created container in bytes) or a string with a units identification char
-('100000b', 1000k', 128m', '1g'). If a string is specified without a units
+('100000b', '1000k', '128m', '1g'). If a string is specified without a units
 character, bytes are assumed as an intended unit.
 
 `volumes_from` and `dns` arguments raise [TypeError](