fix: command in README (#864)

Author: jinseopkim0

cloud-spanner-r2dbc-samples/cloud-spanner-r2dbc-sample/README.adoc

@@ -21,7 +21,7 @@ Run the example `main` method in `SampleApplication.java`, passing in three prop
 For example, to run from command line, you could run the following:
 
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-./mvnw clean install exec:java -DskipTests -f cloud-spanner-r2dbc-sample/ -Dgcp.project=[PROJECT_ID] -Dspanner.instance=[INSTANCE] -Dspanner.database=[DATABASE]
+./mvnw clean install exec:java -DskipTests -Dgcp.project=[PROJECT_ID] -Dspanner.instance=[INSTANCE] -Dspanner.database=[DATABASE]
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 The example creates a table named `BOOKS`.