build(java): readme.sh should be executable (#266)

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Synth log will be available here:
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Source-Link: googleapis/synthtool@0762e8e

synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-talent.git",
-        "sha": "4729b45bb617c9a81f0bf06cee5d939c104df052"
+        "sha": "80f6a9ea03374a91c059602c0f488834d380f75f"
       }
     },
     {
@@ -27,7 +27,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "8a7a3021fe97aa0a3641db642fe2b767f1c8110f"
+        "sha": "0762e8ee2ec21cdfc4d82020b985a104feb0453b"
       }
     }
   ],