chore: regenerate README (#848)

This PR was generated using Autosynth. 🌈


<details><summary>Log from Synthtool</summary>

```
2021-05-26 19:44:07,680 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-storage/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-05-26 19:44:09,191 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

```
</details>

Full log will be available here:
https://source.cloud.google.com/results/invocations/67077738-e103-487e-b0e5-686c35e0614f/targets

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.github/readme/synth.metadata/synth.metadata

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-storage.git",
-        "sha": "c734edf01084419b25825cee1bc1cf9ff8c6369c"
+        "sha": "c71ccc4f5704902b48e458b8225b2ad74a405290"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "4f4b1b9b8d8b52f1e9e4a76165896debce5ab7f1"
+        "sha": "1778de119522ea9c9b18763c2a4577e22a60433c"
       }
     }
   ]

README.md

@@ -45,7 +45,7 @@ If you are using Maven without BOM, add this to your dependencies:
 
 If you are using Gradle 5.x or later, add this to your dependencies
 ```Groovy
-implementation platform('com.google.cloud:libraries-bom:20.4.0')
+implementation platform('com.google.cloud:libraries-bom:20.5.0')
 
 compile 'com.google.cloud:google-cloud-storage'
 ```