vet : add check for tabs in text files (#7678)

Documentation/benchmark.md

@@ -13,9 +13,9 @@ simulate your application:
 ```bash
 $ go run google.golang.org/grpc/benchmark/benchmain/main.go \
     -workloads=streaming \
-  	-reqSizeBytes=1024 \
-  	-respSizeBytes=1024 \
-  	-compression=gzip
+    -reqSizeBytes=1024 \
+    -respSizeBytes=1024 \
+    -compression=gzip
 ```
 
 Pass the `-h` flag to the `benchmain` utility to see other flags and workloads
@@ -45,8 +45,8 @@ Assume that `benchmain` is invoked like so:
 ```bash
 $ go run google.golang.org/grpc/benchmark/benchmain/main.go \
     -workloads=unary \
-  	-reqPayloadCurveFiles=/path/to/csv \
-  	-respPayloadCurveFiles=/path/to/csv
+    -reqPayloadCurveFiles=/path/to/csv \
+    -respPayloadCurveFiles=/path/to/csv
 ```
 
 This tells the `benchmain` utility to generate unary RPC requests with a 25%
@@ -61,15 +61,15 @@ following command will execute four benchmarks:
 ```bash
 $ go run google.golang.org/grpc/benchmark/benchmain/main.go \
     -workloads=unary \
-  	-reqPayloadCurveFiles=/path/to/csv1,/path/to/csv2 \
-  	-respPayloadCurveFiles=/path/to/csv3,/path/to/csv4
+    -reqPayloadCurveFiles=/path/to/csv1,/path/to/csv2 \
+    -respPayloadCurveFiles=/path/to/csv3,/path/to/csv4
 ```
 
 You may also combine `PayloadCurveFiles` with `SizeBytes` options. For example:
 
 ```
 $ go run google.golang.org/grpc/benchmark/benchmain/main.go \
     -workloads=unary \
-  	-reqPayloadCurveFiles=/path/to/csv \
-  	-respSizeBytes=1
+    -reqPayloadCurveFiles=/path/to/csv \
+    -respSizeBytes=1
 ```

scripts/vet.sh

@@ -70,6 +70,9 @@ not git grep "\(import \|^\s*\)\"google.golang.org/grpc/interop/grpc_testing" --
 # - Ensure that no trailing spaces are found.
 not git grep '[[:blank:]]$'
 
+# - Ensure that no tabs are found in markdown files.
+not git grep $'\t' -- '*.md'
+
 # - Ensure all xds proto imports are renamed to *pb or *grpc.
 git grep '"github.com/envoyproxy/go-control-plane/envoy' -- '*.go' ':(exclude)*.pb.go' | not grep -v 'pb "\|grpc "'