docs(constant): fix pluralization

Closes angular#13024

Author: nebulou5

src/loader.js

@@ -188,7 +188,7 @@ function setupModuleLoader(window) {
            * @param {string} name constant name
            * @param {*} object Constant value.
            * @description
-           * Because the constant are fixed, they get applied before other provide methods.
+           * Because the constants are fixed, they get applied before other provide methods.
            * See {@link auto.$provide#constant $provide.constant()}.
            */
           constant: invokeLater('$provide', 'constant', 'unshift'),