docs: Add missing file format to examples

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/channels/merge.py

@@ -17,7 +17,7 @@ class Merge(Receiver[T]):
     can be awaited together, and their results merged into a single stream, by
     using `Merge` like this:
 
-    ```
+    ```python
     merge = Merge(receiver1, receiver2)
     while msg := await merge.receive():
         # do something with msg

src/frequenz/channels/select.py

@@ -34,7 +34,7 @@ class Select:
     For example, if there are two async iterators that you want to
     simultaneously wait on, this can be done with:
 
-    ```
+    ```python
     select = Select(name1 = receiver1, name2 = receiver2)
     while await select.ready():
         if msg := select.name1:
@@ -50,8 +50,9 @@ class Select:
     ```
 
     If `Select` was created with more `AsyncIterator` than what are read in
-    the if-chain after each call to `ready()`, messages coming in the
-    additional async iterators are dropped, and a warning message is logged.
+    the if-chain after each call to [ready()][frequenz.channels.Select.ready],
+    messages coming in the additional async iterators are dropped, and
+    a warning message is logged.
 
     `Receivers` also function as AsyncIterator.
     """

src/frequenz/channels/utils/timer.py

@@ -16,7 +16,7 @@ class Timer(Receiver[datetime]):
     Primarily for use with `channel.Select` calls.
 
     When you want something to happen with a fixed period:
-    ```
+    ```python
     timer = channel.Timer(30.0)
     select = Select(bat_1 = receiver1, timer = timer)
     while await select.ready():
@@ -32,7 +32,7 @@ class Timer(Receiver[datetime]):
 
     When you want something to happen when nothing else has happened in a
     certain interval:
-    ```
+    ```python
     timer = channel.Timer(30.0)
     select = Select(bat_1 = receiver1, timer = timer)
     while await select.ready():