Use correct terminology in style guide (ponylang#3299)

Closes ponylang#3267

[skip ci]

STYLE_GUIDE.md

@@ -553,7 +553,7 @@ fun mod_checked
 - The *file name* should be directly derived from the *principal type name* using a consistent reproducible scheme of case conversion.
   - The *file name* should be the "snake case" version of the *principal type name*. That is, each word in the *principal type name* (as defined by transitions from lowercase to uppercase letters) should be separated with the underscore character (`_`) and lowercased to generate the *file name*. For example, a file that defines the `ContentsLog` type should be named `contents_log.pony`.
   - If the *principal type* is a private type (its name beginning with an underscore character), then the *file name* should also be prefixed with an underscore character to highlight the fact that it defines a private type. For example, a file that defines the `_ClientConnection` type should be named `_client_connection.pony`.
-  - If the *principal type* name contains an acronym (a sequence of uppercase letters with no lowercase letters between them), then the entire acronym should be considered as a single word when converting to snake case. Note that if there is another word following the acronym, its first letter will also be uppercase, but should not be considered part of the sequence of uppercase letters that form the acronym. For example, a file that defines the `SSLContext` type should be named `ssl_context.pony`.
+  - If the *principal type* name contains an initialism (a sequence of uppercase letters with no lowercase letters between them), then the entire initialism should be considered as a single word when converting to snake case. Note that if there is another word following the initialism, its first letter will also be uppercase, but should not be considered part of the sequence of uppercase letters that form the initialism. For example, a file that defines the `SSLContext` type should be named `ssl_context.pony`.
 
 ## Documentation