style: format code with Rustfmt

This commit fixes the style issues introduced in 71e6c26 according to the output
from Rustfmt.

Details: #18

Author: deepsource-autofix[bot]

src/rust/static-btree/src/tree.rs

@@ -139,7 +139,7 @@ impl<K: Key, R: Read + Seek> StaticBTree<K, R> {
     fn lower_bound_index(&mut self, key: &K) -> Result<usize, Error> {
         let b = self.layout.branching_factor;
         let mut node_idx = 0usize; // multiplied by b implicitly per formula
-        // iterate internal layers from root (height-1) down to layer 1, if height>1
+                                   // iterate internal layers from root (height-1) down to layer 1, if height>1
         for h in (1..self.layout.height).rev() {
             let node = self.read_node(h, node_idx)?;
             // linear scan