Fix typos in README files.

Author: Dor1s

lessons/07/README.md

@@ -10,7 +10,7 @@ Here we will be fuzzing [libxml2]. During this lesson we will:
 ### Build the library
 
 ```bash
-tar xzf libxml.tgz
+tar xzf libxml2.tgz
 cd libxml2
 
 ./autogen.sh

lessons/08/README.md

@@ -70,7 +70,7 @@ Run two more fuzzer instances. One with the seed corpus:
 mkdir corpus3
 ./libpng_read_fuzzer -max_len=2048 -max_total_time=300 -print_final_stats=1 \
     -timeout=5 corpus3 seed_corpus
-``
+```
 
 And another one with the seed corpus and the dictionary (super-combo!):