Tweaking an error message to recommend a manifest key prefix with no opening slash

This is to match the fact that, by default, there is no opening slash

Author: weaverryan

lib/config/validator.js

@@ -47,7 +47,7 @@ class Validator {
              * choose your manifestKeyPrefix.
              */
 
-            throw new Error('Cannot determine how to prefix the keys in manifest.json. Call Encore.setManifestKeyPrefix() to choose what path (e.g. /build/) to use when building your manifest keys. This is happening because you passed an absolute URL to setPublicPath().');
+            throw new Error('Cannot determine how to prefix the keys in manifest.json. Call Encore.setManifestKeyPrefix() to choose what path (e.g. build/) to use when building your manifest keys. This is happening because you passed an absolute URL to setPublicPath().');
         }
 
         if (!this.webpackConfig.manifestKeyPrefix) {
@@ -68,7 +68,7 @@ class Validator {
              * For that reason, we force you to choose your manifestKeyPrefix().
              */
             if (outputPath.indexOf(publicPath) === -1) {
-                const suggestion = '/' + publicPath.substr(publicPath.lastIndexOf('/') + 1) + '/';
+                const suggestion = publicPath.substr(publicPath.lastIndexOf('/') + 1) + '/';
 
                 throw new Error(`Cannot determine how to prefix the keys in manifest.json. Call Encore.setManifestKeyPrefix() to choose what path (e.g. ${suggestion}) to use when building your manifest keys. This is caused by setOutputPath() and setPublicPath() containing paths that don't seem compatible.`);
             }