Update client_secret examples so that they are valid (#2985)

The regex of allowed characters for a `client_secret` parameter is `[0-9a-zA-Z.=_-]`.

This PR updates the `client_secret` spec examples, which currently include an invalid character (an apostrophe).

changelogs/client_server/newsfragments/2985.clarification

@@ -0,0 +1 @@
+Correct examples of `client_secret` request body parameters so that they do not include invalid characters.

data/api/client-server/administrative_contact.yaml

@@ -139,7 +139,7 @@ paths:
                   "id_server": "matrix.org",
                   "id_access_token": "abc123_OpaqueString",
                   "sid": "abc123987",
-                  "client_secret": "d0n'tT3ll"
+                  "client_secret": "d0nt-T3ll"
                 }
               }
       responses:
@@ -215,7 +215,7 @@ paths:
             required: ["client_secret",  "sid"]
             example: {
               "sid": "abc123987",
-              "client_secret": "d0n'tT3ll"
+              "client_secret": "d0nt-T3ll"
             }
       responses:
         200:
@@ -272,7 +272,7 @@ paths:
               "id_server": "example.org",
               "id_access_token": "abc123_OpaqueString",
               "sid": "abc123987",
-              "client_secret": "d0n'tT3ll"
+              "client_secret": "d0nt-T3ll"
             }
       responses:
         200: