build(java): readme.sh should be executable (#240)

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Source-Link: googleapis/synthtool@0762e8e

synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-iamcredentials.git",
-        "sha": "1415de05c5aebd471668ea8a5a80df9d7db5a384"
+        "sha": "8a9ad5b1b37b0820161ce1d35d86284e9d957f21"
       }
     },
     {
@@ -19,7 +19,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "8a7a3021fe97aa0a3641db642fe2b767f1c8110f"
+        "sha": "0762e8ee2ec21cdfc4d82020b985a104feb0453b"
       }
     }
   ],