build(java): readme.sh should be executable (#261)

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Source-Link: googleapis/synthtool@0762e8e

synth.metadata

@@ -11,7 +11,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-billingbudgets.git",
-        "sha": "b2b9f849b3b2c74681f9b7a3560a4550bad2f2a5"
+        "sha": "d3476a58b306f5209bcdc5f867daa92ccd9eba29"
       }
     },
     {
@@ -26,7 +26,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "8a7a3021fe97aa0a3641db642fe2b767f1c8110f"
+        "sha": "0762e8ee2ec21cdfc4d82020b985a104feb0453b"
       }
     }
   ],