build(java): readme.sh should be executable (#590)

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Source-Link: googleapis/synthtool@0762e8e

synth.metadata

@@ -11,7 +11,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-bigquerystorage.git",
-        "sha": "6795a5da3625cba03063b16b7b329c88fb8d8118"
+        "sha": "e89c92bf2b88d4020d31fba707ed69d78e9b74bc"
       }
     },
     {
@@ -50,7 +50,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "8a7a3021fe97aa0a3641db642fe2b767f1c8110f"
+        "sha": "0762e8ee2ec21cdfc4d82020b985a104feb0453b"
       }
     }
   ],