Fix typo (#4814)

api/context.go

@@ -140,7 +140,7 @@ func (h handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 			if (h.requireSystemAdmin || h.requireUser) && !h.trustRequester {
 				if r.Header.Get(model.HEADER_REQUESTED_WITH) != model.HEADER_REQUESTED_WITH_XML {
-					c.Err = model.NewLocAppError("ServeHTTP", "api.context.session_expired.app_error", nil, "token="+token+" Appears to bea CSRF attempt")
+					c.Err = model.NewLocAppError("ServeHTTP", "api.context.session_expired.app_error", nil, "token="+token+" Appears to be a CSRF attempt")
 					token = ""
 				}
 			}