Fix markdown list describing Alertmanager matchers

The Markdown variant used on prometheus.io is different from the GitHub one and
requires code blocks to be indented by four spaces to be recognized as part of
a list item vs. terminating the list.

Signed-off-by: Julius Volz <julius.volz@gmail.com>

docs/configuration.md

@@ -856,37 +856,37 @@ Here are some examples of valid string matchers:
 
 1. Shown below are two equality matchers combined in a long form YAML list.
 
-```yaml
-  matchers:
-   - foo = bar
-   - dings !=bums
-```
+    ```yaml
+    matchers:
+      - foo = bar
+      - dings !=bums
+    ```
 
 2. Similar to example 1, shown below are two equality matchers combined in a short form YAML list.
 
-```yaml
-matchers: [ foo = bar, dings != bums ]
-```
+    ```yaml
+    matchers: [ foo = bar, dings != bums ]
+    ```
 
-As shown below, in the short-form, it's generally better to quote the list elements to avoid problems with special characters like commas:
+    As shown below, in the short-form, it's generally better to quote the list elements to avoid problems with special characters like commas:
 
-```yaml
-matchers: [ "foo = bar,baz", "dings != bums" ]
-```
+    ```yaml
+    matchers: [ "foo = bar,baz", "dings != bums" ]
+    ```
 
 3. You can also put both matchers into one PromQL-like string. Single quotes for the whole string work best here.
 
-```yaml
-matchers: [ '{foo="bar",dings!="bums"}' ]
-```
+    ```yaml
+    matchers: [ '{foo="bar",dings!="bums"}' ]
+    ```
 
 4. To avoid any confusion about YAML string quoting and escaping, you can use YAML block quoting and then only worry about the OpenMetrics escaping inside the block. A complex example with a regular expression and different quotes inside the label value is shown below:
 
-```yaml
-matchers:
-  - |
-      {quote=~"She said: \"Hi, all!( How're you…)?\""}
-```
+    ```yaml
+    matchers:
+      - |
+          {quote=~"She said: \"Hi, all!( How're you…)?\""}
+    ```
 
 ## `<victorops_config>`