build(java): readme.sh should be executable (#249)

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Source-Link: googleapis/synthtool@0762e8e

synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-language.git",
-        "sha": "c1eee5b6360ac4e06faf9eca7a10d3cbfb796a88"
+        "sha": "6d2a88980ef9827aeb8e675f3a4c2e7bb488e52e"
       }
     },
     {
@@ -27,7 +27,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "8a7a3021fe97aa0a3641db642fe2b767f1c8110f"
+        "sha": "0762e8ee2ec21cdfc4d82020b985a104feb0453b"
       }
     }
   ],