Adjust config.json to return exercise difficulties to one of 1, 4, 7, 10.

Performed with the following command line:

```shell
jq '.exercises |= map(
    if .difficulty == 3 then
       .difficulty=4
    elif .difficulty == 6 then
       .difficulty=7
    else
        .
    end
)' config.json > config.json.new && \
diff config.json config.json.new ; \
mv config.json.new config.json
```

Author: coriolinus

config.json

@@ -200,7 +200,7 @@
       "slug": "saddle-points",
       "core": false,
       "unlocked_by": null,
-      "difficulty": 3,
+      "difficulty": 4,
       "topics": [
         "vectors",
         "iterators"
@@ -211,7 +211,7 @@
       "slug": "isogram",
       "core": false,
       "unlocked_by": null,
-      "difficulty": 3,
+      "difficulty": 4,
       "topics": [
         "chars",
         "iterators",
@@ -772,7 +772,7 @@
       "slug": "poker",
       "core": false,
       "unlocked_by": null,
-      "difficulty": 6,
+      "difficulty": 7,
       "topics": [
         "lifetimes",
         "struct",
@@ -786,7 +786,7 @@
       "slug": "decimal",
       "core": false,
       "unlocked_by": null,
-      "difficulty": 6,
+      "difficulty": 7,
       "topics": [
         "struct",
         "traits",