Use consistent headings in documentation

Author: ErikSchierboom

exercises/concept/strings/.docs/hints.md

@@ -1,16 +1,16 @@
-### General
+## General
 
 - The [Python documentation for `str`][python-str-doc] has an overview of the Python `str` type.
 
-### 1. Get message from a log line
+## 1. Get message from a log line
 
 - Strings in Python have [lots of convenient instance methods][str-type-methods] for cleaning, splitting, manipulating, and creating new strings. Extracting values from a string could be done by splitting it based on a substring, for example.
 
-### 2. Get log level from a log line
+## 2. Get log level from a log line
 
 - Strings also have methods that help convert letters from lower to uppercase and vice-versa.
 
-### 3. Reformat a log line
+## 3. Reformat a log line
 
 Strings are immutable, but can be combined together to make new strings, or have elements replaced. This goal can be accomplished by using string methods, or operators like `+` or `+=` (which are overloaded to work with strings).
 Python also has a concept of string formatting, like many other languages.

exercises/concept/strings/.docs/instructions.md

@@ -10,7 +10,7 @@ There are three different log levels:
 
 You have three tasks, each of which will take a log line and ask you to do something with it.
 
-### 1. Extract a message from a log line
+## 1. Extract a message from a log line
 
 Implement a function to return a log line's message:
 
@@ -26,7 +26,7 @@ The message should be trimmed of any whitespace.
 'Invalid operation.'
 ```
 
-### 2. Change a message's loglevel.
+## 2. Change a message's loglevel.
 
 Implement a function that replaces a log line's current log level with a new one:
 
@@ -35,7 +35,7 @@ Implement a function that replaces a log line's current log level with a new one
 '[ERROR]: Fatal Error.'
 ```
 
-### 3. Reformat a log line
+## 3. Reformat a log line
 
 Implement a function that reformats the log line, putting the message first and the log level after it in parentheses: