opt: fix readme.md

[Ran-Mori]

command: ./gradlew tasks --all | grep app:assemble
output:

app:assemble - Assemble main outputs for all the variants.
app:assembleAlpha - Assembles main outputs for all Alpha variants.
app:assembleAndroidTest - Assembles all the Test applications.
app:assembleDebug - Assembles main outputs for all Debug variants.
app:assembleMeta - Assembles main outputs for all Meta variants.
app:assembleRelease - Assembles main outputs for all Release variants.
app:assembleUnitTest - Assembles all the unit test applications.
app:assembleAlphaDebug - Assembles main output for variant alphaDebug
app:assembleAlphaDebugAndroidTest - Assembles main output for variant alphaDebugAndroidTest
app:assembleAlphaDebugUnitTest - Assembles main output for variant alphaDebugUnitTest
app:assembleAlphaRelease - Assembles main output for variant alphaRelease
app:assembleAlphaReleaseUnitTest - Assembles main output for variant alphaReleaseUnitTest
app:assembleMetaDebug - Assembles main output for variant metaDebug
app:assembleMetaDebugAndroidTest - Assembles main output for variant metaDebugAndroidTest
app:assembleMetaDebugUnitTest - Assembles main output for variant metaDebugUnitTest
app:assembleMetaRelease - Assembles main output for variant metaRelease
app:assembleMetaReleaseUnitTest - Assembles main output for variant metaReleaseUnitTest

so the command should be ./gradlew app:assembleAlphaRelease instead of ./gradlew app:assembleMeta-AlphaRelease