Use four space indents consistently in all perf-config.json files.

Note that the files generated by the `download` command use four space
indents.

Author: nnethercote

collector/benchmarks/clap-rs/perf-config.json

@@ -1,3 +1,3 @@
 {
-  "category": "primary"
+    "category": "primary"
 }

collector/benchmarks/helloworld/perf-config.json

@@ -1,3 +1,3 @@
 {
-  "category": "primary"
+    "category": "primary"
 }

collector/benchmarks/keccak/perf-config.json

@@ -1,3 +1,3 @@
 {
-  "category": "secondary"
+    "category": "secondary"
 }

collector/benchmarks/unicode_normalization/perf-config.json

@@ -1,3 +1,3 @@
 {
-  "category": "primary"
+    "category": "primary"
 }