oauth2: Use OAuth 2.0 rather than XOAUTH2 in comments

When referring to OAuth 2.0 we should use the official name rather the
SASL mechanism name.

lib/url.c

@@ -1700,7 +1700,7 @@ CURLcode Curl_setopt(struct SessionHandle *data, CURLoption option,
 
   case CURLOPT_XOAUTH2_BEARER:
     /*
-     * XOAUTH2 bearer token to use in the operation
+     * OAuth 2.0 bearer token to use in the operation
      */
     result = setstropt(&data->set.str[STRING_BEARER],
                        va_arg(param, char *));

lib/urldata.h

@@ -915,7 +915,7 @@ struct connectdata {
   char *passwd;  /* password string, allocated */
   char *options; /* options string, allocated */
 
-  char *xoauth2_bearer; /* bearer token for xoauth2, allocated */
+  char *xoauth2_bearer; /* bearer token for OAuth 2.0, allocated */
 
   char *proxyuser;    /* proxy user name string, allocated */
   char *proxypasswd;  /* proxy password string, allocated */

src/tool_cfgable.h

@@ -208,7 +208,7 @@ struct OperationConfig {
 #ifdef CURLDEBUG
   bool test_event_based;
 #endif
-  char *xoauth2_bearer;           /* XOAUTH2 bearer token */
+  char *xoauth2_bearer;           /* OAuth 2.0 bearer token */
   bool nonpn;                     /* enable/disable TLS NPN extension */
   bool noalpn;                    /* enable/disable TLS ALPN extension */
   char *unix_socket_path;         /* path to Unix domain socket */

src/tool_getparam.c

@@ -495,7 +495,7 @@ ParameterError getparameter(char *flag,    /* f or -long-flag */
       case 'b': /* egd-file */
         GetStr(&config->egd_file, nextarg);
         break;
-      case 'B': /* XOAUTH2 Bearer */
+      case 'B': /* OAuth 2.0 bearer token */
         GetStr(&config->xoauth2_bearer, nextarg);
         break;
       case 'c': /* connect-timeout */